The present invention relates generally to the control of variable speed drives. More specifically, the present invention relates to a control technique for a variable speed drive having an active converter and/or multiple inverter stages connected to a common DC link.
A variable speed drive (VSD) can have three stages: a converter stage; a DC link stage; and an inverter stage. The converter stage converts a fixed line frequency, fixed line voltage AC power source to a DC power source. The DC link stage filters the DC voltage from the converter stage. Finally, the inverter stage converts the DC voltage from the DC link stage into a variable frequency, variable voltage AC power source.
A voltage source type VSD may require a large value of capacitance in the DC link stage of the VSD to provide filtering for the converter stage. These capacitors often work in conjunction with an inductor(s) to form a low pass filter in larger VSDs, e.g., VSDs providing more than 100 hp. The inductor(s) may be placed in the DC link stage just before the capacitors, in the AC line that feeds the converter stage, or in both the DC link stage and the AC line feeding the converter stage. The inductors and capacitors forming the low pass filter are passive devices that can be both bulky and expensive and can increase the size and cost of the VSD.
The capacitors used within a VSD are usually of the aluminum electrolytic type and include multilayer “sandwiches” of conducting foil and insulating paper that are rolled into a cylinder and inserted into a cylindrical can. The cans are filled with a liquid electrolyte and two electrical terminals are provided on the can to permit an electrical connection. The conducting foil may be etched to provide improved surface area and lower internal resistance in the capacitor. The internal resistance in the capacitor is the limiting factor with regard to the usage of the capacitor in the DC link stage of the VSD because the power dissipated by the capacitor is limited by the capacitor's ability to reject its own internal power. The internal resistance of the capacitor varies as a function of the frequency of the current flowing through the capacitor and the voltage applied to the capacitor. The internal resistance or equivalent series resistance (ESR) of the capacitor is reduced as the frequency of the current is increased. Capacitor manufacturers want to reduce the capacitor's internal resistance and improve its heat dissipation capability because the internal operating temperature of the capacitor can affect the lifetime of the capacitor. The primary failure mode of an electrolytic capacitor is a loss of electrolyte due to leakage of the electrolyte through the seals of the capacitor can. This leakage of electrolyte from the capacitor can tends to increase as a function of the internal operating temperature of the capacitor leading to a reduction in effective capacitance and eventual wearout or failure of the device.
A voltage source type VSD uses multiple capacitors to provide sufficient filtering for the pulsating DC link ripple current that results from the pulsating converter and inverter currents, which currents converge at the DC link. The size and quantity of the DC link capacitors chosen for a specific application can be based on the magnitude of the ripple current seen at the DC link. Some secondary issues in capacitor selection include the ride-through capability of the VSD in the case of a utility loss of voltage, the magnitude of the resonant frequency associated with the inductive and capacitive elements in the filter of the DC link and the output impedance of the VSD from the motor's standpoint. However, the overriding factor in selecting a capacitor is the life expectancy of the capacitor, which life expectancy is based on the ripple current and the internal operating temperature of the capacitor. The use of multiple inverter stages connected to a single DC link stage can further increase the amount of ripple current in the capacitor by increasing the pulsating inverter currents at the DC link.
Therefore, what is needed is a system and method for controlling a variable speed drive having an active converter and/or multiple inverter stages connected to a common DC link in order to reduce the amount of capacitance required in the DC link or to extend the lifetime of the DC link capacitors in an application.